Output Impedance

[apb]

The impedance chart only lists 6V6,6L6,EL34.

Does the KT66 use the same settings as the 6L6?

What setting for the 6K6?

What tubes were used to measure the voltages quoted in the book?

[Andy]

The KT66 sits at the same impedance as the 6L6, yes.

You can look at some online specs here:

http://tdsl.duncanamps.com/show.php?des=6K6
http://tdsl.duncanamps.com/show.php?des=6L6
http://tdsl.duncanamps.com/show.php?des=6V6

If you look at the Class A S/E (single ended) and move over to the Zout, you will see the impedance for the 6K6 is 9000, and the 6V6 is 8500 where the 6L6 is 4200. You would want to run a 6K6 at the 6V6 position.

I’m not sure what you mean by what tubes were used, do you mean brand? If so, I don’t know what the brands were.

[apb]

I was thinking that a 6k6 will draw different current than an EL34 and that would change the voltages.

My amp with a Sylvania 5Y3 was right at 400v so I have added a 500 ohm resistor before the first filter cap and they are lower but still on the high side.

Also the ripple at the 40ufd cap is about 5v and causes hum if the negative feedback is used.

[apb]

It seems that the output impedance changes with voltage and it looks to be nonlinear?

[Robin]

400v is a workable voltage (and probably sounds great), the 40uf (or even a 30uf or less) cap should not create a hum issue in conjunction with the neg. feedback loop. I would double check the layout and lead dress.

[apb]

I realize it should not cause a problem but it does.

I used a point to point bus grounded layout with no board but there is nothing around the feedback resistor from the output switch. All the AC stuff is at the other end of the amp. The amp is dead quiet without the feedback resistor but hums with feedback connected.

[Robin]

Can you post some photos? It really sounds like a layout or lead dress problem. Try moving the ground for the filter caps to another location and see if it changes the hum.

[apb]

I figured it out.

I floated all the jacks and only tied the ground bus to the chassis at one location. I neglected to tie the bottom of the output transformer back to the ground bus.

It is now dead silent.

[Robin]

Congrats, enjoy the amp!

[Author]

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